A hard-disk drive (HDD) is a non-volatile storage device that is housed in a protective enclosure and stores digitally encoded data on one or more circular disks having magnetic surfaces. When an HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read-write head that is positioned over a specific location of a disk by an actuator. A read-write head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. A write head makes use of the electricity flowing through a coil, which produces a magnetic field. Electrical pulses are sent to the write head, with different patterns of positive and negative currents. The current in the coil of the write head induces a magnetic field across the gap between the head and the magnetic disk, which in turn magnetizes a small area on the recording medium.
While increasing areal density, a measure of the quantity of information bits that can be stored on a given area of disk surface, is one of the ever-present goals of HDD design evolution, the cost per unit of storage is also an ever-present constraint associated with HDD development. The cost per storage capacity (or price per capacity from a user's standpoint) is especially important in large data storage scenarios, such as with archival and backup storage, in which large amounts of data are stored but infrequently accessed. One approach to increasing the storage capacity of an HDD while keeping the cost per capacity at a reasonable point is to utilize an HDD having multiple disk stacks while sharing an actuator among a plurality of the disk stacks. Thus, moving forward with a multiple disk-stack, shared actuator HDD leads to a need for controlling and limiting the travel of the actuator arms across the multiple disk stacks.
Any approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.